The reason I ask if I am stressing the PSU is that since I installed the third 7,200 RPM HDD, every now and again (probably 5 times a day), my mouse disconnects (Windows plays Device Disconnect sound) and reconnects a second or two later (Windows plays Device Connect sound).

It could be a co-incidence. However, I am wanting to install a fourth 7,200 RPM HDD soon and I want to make sure I'm not already stressing the PSU as it is.

500w output is it's PEAK output, no PSU should run at PEAK, in fact, they won't run at PEAK continiously.
Plus you've got to account for plus/minus 10% and you shouldn't be running a PSU at more than 90% or 95% capacity of this AT MOST.
10% off 500w = 450w, so if you're using 431w I would strongly recommend getting a more powerful PSU - or plug the internal drives into a seperate much smaller PSU.

500w output is it's PEAK output, no PSU should run at PEAK, in fact, they won't run at PEAK continiously.Plus you've got to account for plus/minus 10% and you shouldn't be running a PSU at more than 90% or 95% capacity of this AT MOST.10% off 500w = 450w, so if you're using 431w I would strongly recommend getting a more powerful PSU - or plug the internal drives into a seperate much smaller PSU.